Global Warming vs. Other Causes of Shifts in Bird Ranges

Hockey and Midgley (2009) write that "in the influential fourth assessment report of the Intergovernmental Panel on Climate Change, Rosenzweig et al. (2007) tested several thousand time-series data sets for changes in species behavior and geographic range consistent with climate change, reaching the conclusion that it is very likely that climate change is driving changes in natural biological systems." However, they say that "the use of such large data sets in meta-analyses may discourage the close inspection of observations and result in naively misattributing observed shifts to climate when other explanations may be more parsimonious."

To test this hypothesis, Hockey and Midgley, as they describe it, "collated information about recent range changes in South African birds, specifically indigenous species that have colonized the Cape Peninsula, at the south-western tip of Africa in the Western Cape province, since the 1940s," where they say there have been "widespread anthropogenic changes of many kinds to the landscape, including urbanization, commercial afforestation and the introduction and spread of invasive alien trees, most of which occurred before climate change accelerated in the 1970s."

The two researchers found that the colonization events "concur with a 'climate change' explanation, assuming extrapolation of Northern Hemisphere results and simplistic application of theory," but they found that, "on individual inspection, all bar one may be more parsimoniously explained by direct anthropogenic changes to the landscape than by the indirect effects of climate change." And they add that "no a priori predictions relating to climate change, such as colonizers being small and/or originating in nearby arid shrub-lands, were upheld."

In discussing their findings, the South African scientists say their work suggests that either "observed climate changes have not yet been sufficient to trigger extensive shifts in the ranges of indigenous birds in this region, or that a priori assumptions are incorrect." Either way, as they continue, "this study highlights the danger of naive attribution of range changes to climate change, even if those range changes accord with the predictions of climate-change models," because "misattribution could distract conservationists from addressing pressing issues involving other drivers of biodiversity change such as habitat transformation, and obscure important lessons that might be learned from the dynamics that pertain to such changes."